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The seat belt warning buzzer is customizable and can be turned off for both sides. This can only be done with a Toyota Techview scan tool that accesses the body ECU. If you ask the dealer then can do this for you. They may charge you .5 in labor but it is not hard and take little time to do.
Thank you for your time. The procedure below has not worked for me, just FYI.
Gen 3 Prius DISABLE REVERSE BEEP AND SEAT BELT CHIME (WITHOUT DEALER)
HOW TO DISABLE REVERSE BEEP AND SEAT BELT CHIME (WITHOUT DEALER) 1. With your foot on the brake pedal, press the start button to turn the car on. 2. Press the trip/odometer button until "ODO" appears on the dashboard (if it's already in that mode when you start the car, press the button three times to cycle through the "Trip" modes until you're back to "ODO" mode). 3. Press the power button to turn the car off. 4. With your foot on the brake pedal, press the start button to restart the car. 5. Press and hold the trip/odometer button within six seconds of starting the car. Continue holding the ODO button for at least 10 seconds, but do NOT release the ODO button until after the next step. (This is easiest if you use the simple method of counting, "one-one-thousand, two-one-thousand, three-one-thousand," etc., up to "ten-one-thousand".) 6. While still holding the ODO button from the step before, but after 10 seconds have elapsed, shift the car into reverse and quickly press the park button. Now release the trip/odometer button. 7. Note that the trip/odometer display will show "b on" instead of miles. 8. Press the trip/odometer button until the display shows "b off." 9. Press the Power button to turn the car off. 10. When you start the Prius back up, the reverse beep will be gone. 11. To re-enable reverse beep, repeat the procedure. "b off" will be displayed in step 7. Toggle it to "b on" in step 8. 12. Please note that this procedure will work ONLY on a North American model Prius, not on any other model. 13. To disable the seat belt beep use same steps as above, but instead of shifting into reverse, fasten the driver's seat belt and then quickly press "park".